1. Field of the Invention
This invention relates to a corrosion-resistant/pressure-resistant member made of polycrystalline alumina and an arc tube to be used for a high intensity discharge lamp.
2. Description of the Related Art
Metal halide with high luminous efficacy and excellent color rendering is sealed as a luminous substance into high intensity discharge lamps such as mercury lamps or high-pressure sodium lamps. Since this metal halide is vaporized in arc tubes to have high pressure and strong corrosiveness, arc tubes which are made of polycrystalline alumina with excellent pressure resistance and corrosion resistance are widely used.
However, a grain boundary is easily attacked by a corrosive substance, and as the crystal grains are smaller, the grain boundary increases, namely, the corrosion resistance is occasionally deteriorated. Meanwhile, from a viewpoint of the strength, since the grain boundary becomes a kind of a joint portion, as a number of the boundaries is larger, namely, the crystal grains are smaller, the strength is improved. For this reason, the grains should be small in order to obtain sufficient strength, and the grains should be large in order to obtain corrosion resistance. Therefore, it is difficult that they are compatible.
In addition, since alumina members have excellent corrosion resistance, they are widely used for chemical-resistant containers or appliances which are resistant to chemicals such as acid, but such use in portions which require the chemical resistance and strength is limited due to the above reason.
Further, in the case of the arc tube made of polycrystalline alumina, the total transmittance is 95% and the In-line transmittance is about 3%. In the case where a quartz glass is used, since its total transmittance is low (in comparison with 100%), a post-process which heightens the transmittance is executed in such a manner that surface of the arc tube is polished by mechanical polishing and chemical polishing after calcining so as to be smoothed. As a result, the total transmittance becomes 98% and the In-line transmittance becomes 20%, for example.
The alumina composing the arc tube is formed so that alumina crystals with a normal average grain diameter of 20 to 30 xcexcm are connected with each other, and its surface is rough microscopically. For this reason, the surface is polished so that the transmittance can be heightened. However, since such polishing work is done on a curved surface and the arc tube is small, this is a troublesome step.
The present invention is devised in order to solve the above problem, and its object is to provide a corrosion-resistant alumina member which has excellent corrosion resistance and pressure resistance, namely, strength, and to provide arc tubes for high intensity discharge lamps made of satisfactory polycrystalline alumina which have high corrosion resistance and high strength and in which light transmittance is excellent without executing a polishing step such as mechanical polishing or chemical polishing.
In order to solve the above problems, a corrosion-resistant alumina member in accordance with the invention comprises polycrystalline alumina, wherein an average crystal grain diameter of a surface is two to ten times as large as an average crystal grain diameter of a portion including a center line of a thickness.
When such a difference in grain diameter is given between the surface and the inside, the grain diameter of the surface as an exposed portion is large so that the corrosion resistance is improved. Meanwhile, since the grain diameter of the inside is small and is maintained in a state at the time of calcining, the strength is not deteriorated.
The alumina member in accordance with the invention has a transmittance in the thickness of 1 mm of not less than 85%. As a result, a lowest light transmitting property is secured, and the invention can be used as a window member or a container for equipment requiring corrosion resistance and high strength.
Further, arc tubes for high intensity discharge lamps in accordance with the invention comprise the corrosion-resistant alumina member of which transmittance in the thickness of 1 mm is not less than 85%. In the arc tubes for high intensity discharge lamps in accordance with the invention, an average crystal grain diameter of a portion including a centerline of a thickness is 10 xcexcm to 100 xcexcm. According to this structure, the invention can be used as arc tubes without executing mechanical polishing or chemical polishing.
In addition, in the arc tube for a high intensity discharge lamp in accordance with the invention, the thickness of the corrosion-resistant alumina member is 0.1 mm to 7 mm. When the thickness is controlled between theses values, the arc tube in which the total transmittance is not less than 98% and the In-line transmittance is not less than 5% can be obtained. As a result, the satisfactory arc tube can be formed without executing mechanical polishing or chemical polishing which was conventionally executed in an arc tube made of polycrystalline alumina in order to improve transmittance.